How to Estimate Paint for Interior Doors

Count the square footage first

Painting a door is just a small area problem. If you know the door size, how many sides you’re painting, and how many coats you want, you can estimate the paint on paper before you buy anything.

The basic idea is simple:

• Find the area of one door face: width × height
• Multiply by the number of doors
• Multiply by the number of sides painted
• Multiply by the number of coats to get coverage
• Divide by the paint’s coverage per gallon
• Round up so you don’t run short

That last step matters. Paint calculators are only useful if they leave you with enough paint to finish the job, not a number that looks neat and fails in the real world.

The math, in plain English

Start with one door.

If a door is 30 inches wide and 80 inches tall, its face area is:

30 × 80 = 2,400 square inches

That is not a useful unit for buying paint, so convert it to square feet:

2,400 ÷ 144 = 16.7 square feet

Why 144? Because there are 144 square inches in a square foot.

Now scale that up:

• Number of doors: how many doors you’re painting
• Sides painted: 1 for one face, 2 for both faces
• Coats: each coat counts as another full pass over the same area

So the total area is:

door width × door height × number of doors × sides painted ÷ 144

That gives you total square feet of area.

Then you compare that to the paint can’s coverage rate. Many paints list coverage on the label, and that number is the one that matters. If a gallon covers 350 square feet, then:

coverage ÷ 350 = gallons needed

If the result is 1.18 gallons, you do not buy 1.18 gallons. You round up to the next quarter gallon, because that is how this calculator keeps you from coming up short.

A quick example

Say you have 3 interior doors, each 32 inches wide and 80 inches tall. You’re painting both sides and using 2 coats.

1. One door face:

   • 32 × 80 = 2,560 square inches
   • 2,560 ÷ 144 = 17.8 square feet

2. All doors, one side, one coat:

   • 17.8 × 3 = 53.4 square feet

3. Both sides:

   • 53.4 × 2 = 106.7 square feet

4. Two coats:

   • 106.7 × 2 = 213.3 square feet

If your paint covers 350 square feet per gallon:

• 213.3 ÷ 350 = 0.61 gallons

That means you need a little over half a gallon. But because paint is sold in fixed sizes and the calculator rounds up to the nearest quarter gallon, the buy amount will be rounded to a practical purchase size.

What to enter in the calculator

Use the actual door size, not the rough opening in the wall.

• Number of doors: total doors in the project
• Door width: the width of one door in inches
• Door height: the height of one door in inches
• Sides painted: 1 or 2, depending on whether you’re painting one face or both
• Coats: the number of full coats you plan to apply

If your doors are different sizes, calculate them separately. Mixing sizes into one average is how people end up with the wrong amount of paint.

Why quarter-gallon rounding is useful

Paint is not sold in infinite precision. You buy real containers, not decimal points. Rounding up to the nearest quarter gallon is a practical middle ground:

• It avoids underbuying
• It reduces the chance of stopping mid-project
• It gives you a little buffer for touch-ups and roller loss

That buffer is not waste. It is insurance against a second trip to the store.

What the numbers do not cover

This is where people get tripped up.

The calculator gives you area, coverage, and paint quantity. It does not know everything about your door or your paint job.

It does not account for:

• Raised panels, trim profiles, or decorative molding
• Heavy texture or very porous surfaces
• Primer needs on bare wood, patched spots, or color changes
• Extra paint lost in the tray, brush, or sprayer
• Different coverage rates from different paint brands
• Hardware removal or masking around hinges and handles

Also, not every door should be treated like a flat rectangle in the real world. A paneled door has more surface detail than a plain slab door. If you want a safer estimate for a detailed door, a little extra paint is smarter than a too-tight calculation.

Where people usually get the calculation wrong

The biggest mistakes are boring, which is why they happen so often:

• Using feet for one dimension and inches for the other
• Forgetting to divide by 144 when converting square inches to square feet
• Counting one side when both sides are being painted
• Forgetting that two coats doubles the area to be covered
• Assuming every gallon covers the same amount without checking the label
• Buying exactly the calculated amount instead of rounding up

The calculator handles the arithmetic. Your job is just to enter the right inputs.

The short version

Measure one door in inches, choose the number of doors, sides, and coats, then let the calculator turn that into total area and paint to buy.

The formula is straightforward:

width × height × doors × sides ÷ 144 = area

area × coats = coverage

Then divide by your paint’s coverage rate and round up to the nearest quarter gallon. That is the cleanest way to avoid buying too little.